1. Field of the Invention
The present invention relates to current measuring systems and techniques. More specifically, the present invention relates to systems and techniques for accurately measuring current in Pulse Width Modulated (PWM) amplifier driven inductive loads.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
The torque of a three-phase brushless motor employed to drive a load must be accurately controlled in order to precisely position the load. The torque of the motor is a direct function of the current in the motor armature windings. Therefore, the motor armature current must be accurately measured and controlled in order to control the torque of the motor.
The three-phase brushless motor can be utilized to drive, for example, the reels of a tape drive system and can include three separate wye-connected inductive windings. A three-phase full bridge power amplifier directs current through the motor by pulse width modulating (PWM) switches that impress supply rail voltage across the appropriate armature terminals. Each PWM switch is connected across a flyback diode which provides a path for armature current when no path is provided by the switches. By closing the appropriate combination of switches across the diodes as is known in the art, the amplifier can provide current flow in any direction under all possible phase combinations.
The three-phase full bridge power amplifier can operate in either the continuous conduction mode (CCM) or the discontinuous conduction mode (DCM) of operation. In the CCM, two unique conduction states (S1, S2) exist during each PWM cycle for the amplifier to toggle between. However, a third "idle" state (S3) exists in the PWM cycle during the DCM of operation. State S1 exists at the beginning of each PWM cycle when the switches connect the armature windings across the power supply rails. Armature current increases nearly linearly during state S1.
State S2 is initiated when all PWM switches are open circuited. The armature inductance forces current to continue to flow through the windings in the same direction during state S2. Since the PWM switches are open circuited, the only available current path is from electrical ground through the flyback diode network, the armature inductor and back to the power supply rails. Armature current decreases nearly linearly with time usually at a faster rate than it increased during state S1. Other more sophisticated power amplifiers intended to operate in CCM only, bypass the flyback diodes by switching on appropriate power Field Effect Transistors (FET's) to conduct in the reverse direction during state S2. This design reduces power loss to the diode forward voltage drop.
The idle state S3 begins only if the armature current has decayed all the way to zero during state S2. The switches remain open until the end of the PWM period and no current flows in the circuit until the beginning of the next PWM cycle. The armature current is discontinuous which defines DCM operation. At high armature current and wide pulse width, state S3 may never exist. The next PWM cycle starts before armature current decays all the way to zero. The beginning of the next cycle places the PWM amplifier back into state S1. Thus, the armature current begins to rise again before it ever decays to zero. This condition defines the CCM of operation as the armature current is continuously present throughout the entire PWM cycle. In CCM operation, the PWM amplifier is never idle since it only toggles between states S1 and S2. However, in DCM operation, armature current does reach zero and the PWM amplifier cycles through each of the three states every PWM cycle.
In graphical illustrations known in the art, state S1 appears as a ramp-up function and state S2 appears as a ramp-down function. Current does not flow in state S3. The duty cycle or fraction of time that the PWM amplifier operates in states S1, S2 and S3 is represented by D1, D2 and D3, respectively. The armature current, defined as a positive value, can decay to zero but cannot change polarity during any single PWM cycle. However, power supply current does alternate during a PWM cycle. The power supply current is positive during state S1 and negative during state S2 when the motor returns power to the power supply rails.
In order to control the motor torque, the armature current must be sensed. Known systems and techniques to sense the motor armature current include sense resistor circuits and pilot sensing circuits.
Sense resistor circuits are typically utilized in applications which require accurate current measurements. In sense resistor circuits, sense resistors are placed in the armature current carrying path of the PWM amplifier. The sense resistors produce voltage signals proportional to armature current. The voltage signals, when amplified and conditioned, represent armature current to the system. The sense resistor current changes polarity between states S1 and S2 just as the supply current changes polarity. To obtain average current throughout a full PWM cycle, a switching network or dual sense resistor configuration is required. Therefore, pairs of sensing resistors are used to individually connect the PWM switches and the flyback resistors to ground. The sense resistor tied to the switch carries armature current during state S1 while the sense resistor tied to the flyback diodes carries armature current during state S2. The voltage drops generated across the two sense resistors are summed algebraically and averaged over the total PWM cycle. An integrating operational amplifier circuit is often utilized to perform this function.
Sense resistor circuits have certain characteristics that limit their use in integrated motor amplifier circuits. For example, relatively high power is dissipated in the sense resistors which is incompatible with integrated circuit (IC) design. High power dissipating elements are to be avoided in IC design to minimize damage to the circuitry. When necessary, they are best implemented off-chip. However, off-chip sense resistors require additional high current input/output pins on the IC which is also undesirable. The transition of current from one sense resistor to another is nearly instantaneous between states S1 and S2. The lead and body inductance of off-chip sense resistors creates damaging voltage transients when driven by rapid transitions in current.
Integrating the voltage transients that occur across the inductive sense resistors adds error to the output signal. The onset of states S1 and S2 causes inductive voltage spikes to occur across the sense resistors. The voltage-time product of these inductive voltage spikes adds to the perceived armature current even though the voltages do not represent actual current through the sense resistor. Even non-inductive resistors suffer from error producing transient effects. The capacitance of the lower switch FET's gate carries displacement current. Switching the FET on or off causes gate-source current spikes to pass through the sense resistor. This gate-source current does not represent motor armature current. Susceptibility to this offset error increases with PWM frequency, power FET capacitance and low armature current.
Sense resistor signals with their attendant sharp transients require high bandwidth operational amplifiers for accurate signal processing. Intelligent power IC technologies do not provide high gain-bandwidth operational amplifiers. Besides the gain-bandwidth requirements, operational amplifiers must process both positive and negative going signals from the sense resistors. This requirement necessitates dual power supply rails that increase system cost and complexity.
Pilot sensing circuits are also known as a means to sense motor armature current which is then used to control motor torque. Conventional pilot sensing circuits are employed in applications to detect peak current. Pilot sensing circuits are often used to measure current through PWM switches. As is known in the art, a pilot circuit is a miniature copy of a PWM switch that produces a scaled down replica of the switch current. Therefore, conventional pilot circuits are useful to detect load overcurrent conditions in the armature windings. In a conventional pilot sensing circuit, the output signal indicates whether the sum of the currents through the pilot circuits exceeds a threshold value.
Conventional pilot sensing circuits also have certain characteristics that limit their use in integrated motor amplifier circuits. For example, errors in pilot circuit currents arise from technology, circuit and application effects. Many IC power technologies do not support accurate power device scaling. Further, scaled pilot circuits have built-in errors due to geometrical effects. Even accurately scaled pilot sensing circuits suffer errors due to terminal voltage differences. Ideal pilot circuits produce accurate current outputs only when the voltage bias at all corresponding terminals of the PWM switch and pilot circuits are identical. Conventional pilot circuit configurations violate this condition for at least one terminal. Advances in pilot sensing circuit design have addressed these problems.
Another problem is that conventional pilot sensing circuits are not accurate for average current measurements. The PWM switches and the associated pilot sensing circuits conduct current only during state S1. Thus, when the PWM switches are open circuited during states S2 and S3, current does not flow through the pilot circuits. Average armature current must account for the entire PWM cycle. However, pilot sensing circuits provide an output signal only during a portion of the PWM cycle. Thus, integration of the pilot current alone ignores states S2 and S3 and cannot accurately reflect the average armature current.
Hence, a need remains in the art for a system and technique that compensates for the idle state S3 during DCM operation of the PWM amplifier which will ensure the measurement of true average motor armature current.